This invention relates generally to a magnetic recording medium for use in magnetic disks, and the like. More particularly, the present invention relates to a magnetic recording medium which will be suitable for high density recording, has a high S/N (signal-to-noise) ratio but less fluctuation of reproduction output and has high performance and high reliability.
A magnetic recording medium using a metallic magnetic thin film for a magnetic layer has been proposed in the past as a magnetic recording medium for high density recording as disclosed, for example, in Japanese Patent Publication No. 33523/1979. Various methods such as sputtering, vacuum evaporation, plating, ion plating, ion beam sputtering, and so forth, are available as the method for forming this magnetic recording medium. Demands for high density recording have been increasing recently and attempts have been made to provide a continuous thin film medium using a metallic magnetic thin film for provide practical application.
However, the continuous thin film medium exhibits fluctuation called "modulation" in reproduction output, as discussed in F. A. Hill et al.; "J. Vac. Sci. Technol., A4(3), May/Jun 1986, pp. 547-549". This modulation occurs because the magnetic anisotropy develops in one direction of a disk so that when magnetic characteristics in a circumferential direction are taken into consideration, magnetic characteristics of a magnetic layer change from position to position of the disk, and this modulation is not desirable for practical application. It is known that the magnitude of modulation depends on the film deposition condition of the magnetic layer and underlayer as discussed in the article described above and modulation can be reduced by selecting a suitable film deposition condition. However, the article only makes mention of modulation but does not at all examine definite characteristics important for magnetic recording such as reproduction output, recording density, output noise characteristics (S/N ratio), and the like.
As to the read and write characteristics of a magnetic recording medium, on the other hand, it is known that a high S/N ratio can be accomplished by laminating NiP and Au or NiP and Cu on a substrate by plating to form an underlayer, as disclosed in Japanese Patent Laid-Open No. 157130/1980. It is also known from Japanese Patent Laid-Open No. 62825/1983 (corresponding to U.S. Pat. No. 4,610,911) that high coercive force can be accomplished by film-deposition continuously of Si and Bi as the underlayer on a substrate by vacuum evaporation, and forming further a Co-Ni alloy layer. Furthermore, Japanese Patent Laid-Open No. 111323/1985 teaches that satisfactory magnetic characteristics can be accomplished by forming continuously a Cr underlayer and a Co-Pt magnetic layer by sputtering. In all cases, it cannot be said that these prior art references sufficiently examine the uniformity of read and write characteristics such as modulation.
As described above, the prior art technique does not at all make any study of the optimization of overall read and write characteristics of a magnetic recording medium such as reproduction output change called modulation, the S/N ratio, recording density, and so forth.